Reflectance measurement of macular pigment using multispectral imaging

ABSTRACT

Methods and apparatus are provided for accurately imaging, assessing and measuring a patient&#39;s macular pigment. A multiband filter is employed in combination with a color digital fundus camera to provide a method that operates with a single imaging exposure. The multiband filter has bandpass regions within spectral ranges of the red, green and blue detectors of the CCD array employed within the fundus camera, the bandpass regions being sufficiently sharply defined so as to avoid regions where the CCD detector responses spectrally overlap. This provides three discrete channels of grayscale data corresponding to the bandpass regions of the multiband filter, which can be used to calculate macular pigment topographically. Methods are also disclosed for calculating the optical density of the macular pigment and advantageously displaying the resulting data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of my U.S.Provisional Patent Applications each entitled “Reflectance Measurementof Macular Pigment Using Multispectral Imaging, Ser. Nos. 60/913,885,filed Apr. 25, 2007, and 60/926,354, filed Apr. 26, 2007, which are eachincorporated herein by reference, and further incorporates herein byreference my U.S. patent application entitled “AutofluorescencePhotography Using a Fundus Camera,” Ser. No. 11/742,672 filed on May 1,2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is in the field of opthalmology, and moreparticularly concerns measuring macular pigment by imaging the maculawith a plurality of wavelengths simultaneously.

2. Background of the Related Art

There are a number of circumstances that arise in opthalmology in whichit is necessary or desirable to measure macular pigment. Increasedingestion of lutein and zeaxanthine has been associated with increasedrisk of macular degeneration in some studies.³ (All bibliographicreferences are listed by number in Table I.) Supplements to increase theamounts of these pigments are a popular means to try to decrease therisk of macular degeneration. The absorption of these pigments appearsto vary from individual to individual.^(4,5) Some studies suggest acorrelation between serum levels of lutein and zeaxanthine and reducedrisk, while others do not.⁶⁻⁸ However, the relationship between bloodlevels of these pigments and ultimate deposition in the macula is notknown with certainty. In some circumstances there appears to beincreased metabolism of macular pigments. Smoking, for example, isassociated with decreased levels of macular pigment.⁹ Obesity is alsoassociated with decreased pigment,¹⁰ perhaps because these pigments arefat soluble.

Having a method to measure macular pigment would help in evaluating andtreating older patients at risk for macular degeneration. There areseveral ways to measure macular pigment. Heterochromatic flickermatching is a psychophysical test where the person has to adjust thebrightness of colors so that they match. This is difficult for somepatients to do, particularly as they get older. This test measures onlyone point in the macula, the one used to visualize the colors. Thetester does not know what point is being measured. It is likely that thedistribution of macular pigments in the macula is important, not justthe amount at the one point being measured. Raman spectroscopy, anothermeasurement method, uses inelastic scattering of the macular pigments tomeasure their presence. Raman spectroscopy measures one point in the eyeand the exact location of this point is not known to the tester.

Additional methods used to measure the amount of macular pigment usuallyuse more than one wavelength. Reflectance photography uses twowavelengths. The first wavelength is blue, in the region of maximalabsorbance of the macular pigment, which is around 465 nm. The secondwavelength chosen is to be somewhat longer, but out of the range ofmaximal absorption of the macular pigment, which declines to low levelsat wavelengths longer than 530 nm. The ratio of the optical absorptionat the two test wavelengths can be used to calculate the opticaldensity.

Autofluorescence photographic approaches also can use two wavelengths toestimate the amount of macular pigment present. In this method twowavelengths are used to stimulate autofluorescence, one blue and anotherthat is usually in the green portion of the spectrum. The blue lightwould be blocked by the macular pigment while the green light would not.The ratio of the induced autofluorescence would be indicative of theamount of macular pigment.

There are advantages to both the reflectance photography andautofluorescence methods. The reflectance method does not rely on theunproven assumption that the difference in the amount of fluorescencecaused by the two wavelengths is only due to the presence of blue lightabsorbing pigments. Reflectance methods may, however, containartifactual errors in that the surface of the retina may reflect lightin inverse proportion to the wavelength used. This would increase theamount of blue light reflected, causing an underestimation of the amountof macular pigment present. An advantage of both methods is that a mapof the amount of macular pigment is produced, not just a point estimate.Since macular degeneration involves the entire macula, knowledge of thetopographical distribution of macular pigment would likely be moreuseful. On the other hand, these methods have a common disadvantage.Because two wavelengths are used, two different pictures need to betaken of the fundus. Even if these pictures are taken in close proximityto each other there is invariably movement of the eye and camera. Thismovement causes slight shifts in the field of view, magnification, andlighting of the fundus. It is possible to correct for these changes withthe use of digital image processing software, but the correctionsnecessary take time, potentially introduce artifacts, and requirespecialized software.

TABLE I REFERENCES 1 Bhosale P, Bernstein PS. Synergistic effects ofzeaxanthin and its binding protein in the prevention of lipid membraneoxidation. Biochim Biophys Acta. 2005; 1740: 116-21. 2 Davies NP,Morland AB. Macular pigments: their characteristics and putative role.Prog Retin Eye Res. 2004; 23: 533-59. 3 Seddon JM, Ajani AU, SperdutoRD, et al. Dietary carotenoids, vitamins A, C, and E, and advancedage-related macular degeneration. Eye Disease Case- Control Study Group.JAMA 1994; 272: 1413-20. 4 Curran-Celentano J, Hammond BR Jr, Ciulla TA,et al. Relation between dietary intake, serum concentrations, andretinal concentrations of lutein and zeaxanthin in adults in a Midwestpopulation. Am J Clin Nutr. 2001; 74: 796-802. 5 Hammond Jr., B. R.,Fuld, K. and Curran-Celentano, J., 1995. Macular pigment density inmonozygotic twins. Invest. Ophthalmol. Vis. Sci. 36, pp. 2531-2541. 6Mares-Perlman JA, Brady WE, Klein R, Klein BE, Bowen P, Stacewicz-Sapuntzakis M, et al. Serum antioxidants and age-related maculardegeneration in a population-based case control study. Arch Ophthalmol.1995; 113: 1518-1523. 7 Mares-Perlman JA, Fisher AI, Klein R, Palta M,Block G, Millen AE, el al. Lutein and zeaxanthin in the diet and serumand their relation to age-related maculopathy in the third nationalhealth and nutrition examination survey. Am J Epidemiol 2001; 153:424-32. 8 Dasch B, Fuhs A, Schmidt J, et al. Serum levels of macularcarotenoids in relation to age-related maculopathy: the Muenster Agingand Retina Study (MARS). Graefes Arch Clin Exp Ophthalmol. 2005; 243:1028-35. 9 Hammond Jr., B. R., Wooten, B. R. and Snodderly, D. M., 1996.Cigarette smoking and retinal carotenoids: implications for age-relatedmacular degeneration. Vision Res. 36, pp. 3003-3009. 10 Hammond Jr., B.R., Ciulla, T. A. and Snodderly, D. M., 2002. Macular pigment density isreduced in obese subjects. Invest. Ophthalmol. Vis. Sci. 43, pp. 47-50.

SUMMARY OF THE INVENTION

The present invention overcomes a number of shortcomings of the priorart in measuring macular pigment by using a multispectral imagingtechnique that can provide such a measurement based on taking only asingle image.

In one embodiment, a multiband filter is employed in combination with acolor digital fundus camera. In that embodiment, the multiband filterhas bandpass regions within spectral ranges of the red, green and bluedetectors of the CCD array internal to the fundus camera, the bandpassregions being sufficiently sharply defined so as to avoid regions wherethe CCD detector responses spectrally overlap. This provides threediscrete channels of grayscale data corresponding to the bandpassregions of the multiband filter, which can be used to calculate macularpigment topographically.

The invention also concerns the use of a system comprising, in oneembodiment, a fundus camera having a CCD and a Bayer filter, a multibandfilter, and a processor processing, storing, and displaying theresultant images, as more fully described in the detailed description.

Other features and advantages of the invention will be apparent from thedescription of the drawings and the detailed description which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a Bayer filter and a CCD as used in the prior art.

FIG. 2 shows an illustrative frequency response of a typical Bayerfilter used in digital cameras.

FIG. 3 is a simplified schematic diagram of a fundus camera-based systemfor macular imaging in accordance with the invention.

FIG. 4 shows the idealized frequency response of a multiband filter ofone embodiment of the present invention.

FIG. 5 shows a frequency response of a filter system where the multibandfilter is coupled with a digital camera having a CCD with a Bayerfilter.

FIG. 6 is schematic representation of an image in accordance with theinvention showing certain regions used for measurements andcalculations.

FIG. 7 is an example of a multispectural photograph taken with oneembodiment of the present invention.

DETAILED DESCRIPTION

The following detailed description describes a number of preferredembodiments of various aspects of the invention. These embodiments areexamples only of the full scope of what is enabled by this disclosure,and are not intended to limit the scope of the claims.

The present invention uses multispectral imaging, in which a pluralityof light wavelengths are concurrently measured to obtain informationabout a target. In one embodiment, a reflectance method is used, inwhich multiband filters are used to simultaneously record specificwavelengths.

Multispectral imaging is commonly used in satellite imaging, but has notbeen widely applied to the eye. Often, multispectral imaging isaccomplished by the use of multiple bandpass filters, with each selectedbandwidth being imaged concurrently by different detectors. The use ofmultiple detectors would be difficult and expensive in clinical imagingof patients' eyes. Fundus cameras at one time used film for photographicdocumentation. With development of improved high resolutionmonochromatic charged coupled devices (CCDs), monochromatic film use wasreplaced with monochromatic digital imaging. It was common to use colorslide film to record color images. Later development of sensitive,high-speed color digital cameras has led to the replacement of colorfilm with electronic detectors in imaging the fundus.

Digital color cameras use monochromatic CCDs with a varying pattern ofcolor filters in front of the individual elements of the CCD, so as toprovide a color separation for the various monochromatic detectors. Inthis way, a plurality of grayscale channels can be output, eachcorresponding to one of the separated colors.

The most common method for using monochromatic CCDs in digital colorcameras is to use a filter having a “Bayer” pattern (known as a “Bayerfilter”). A schematic representation of a CCD with a Bayer filter isshown in FIG. 1. As shown in FIG. 1, the Bayer filter 104 is overlaid onCCD array 102 comprising individual monochromatic CCD elements 103 etc.The Bayer filter 104 has a checkerboard pattern of red, green, and bluefilters, each positioned in front of a CCD element. The red, green andblue components, respectively, of incoming light 106 are passed by therespective red, green and blue filter elements 107, 108 and 109, toilluminate the respective underlying sets 110, 111 and 112 of CCDelements. Thus, the spectral sensitivity of each monochromatic CCDelements is modified by the filter element overlaying it. The three setsof CCD elements, 110, 111 and 112 produce three color-separatedgrayscale channels, 1A, 1B and 1C. By maintaining a representation ofthe coordinates of each CCD element in the outgoing signal, topographic,color-separated grayscale signals may be obtained in raster or othersuitable form.

One problem, however, is that the color separation provided by theabove-described arrangement is not very sharply defined. Since thedesign goal of a digital color camera is to produce accurate andpleasing color images, the red, green, and blue filters are notparticularly specific and show significant overlap. Thus, the grayscaledata obtained from a typical digital color camera is not sharply definedin the red, green and blue color ranges.

The spectral characteristics of a typical digital color camera employinga CCD array with a Bayer filter is shown in curves 202, 204 and 206 ofFIG. 2. It can be seen that there are significant spectral overlapregions between the Bayer filter elements, at 208 and 210. Theabsorption curve 214 for macular pigment is superimposed for comparison.As can be observed, while centered in the blue region 206, significantportions of this absorption curve also lies in the passband 204 of thegreen Bayer filter. This overlap would reduce the accuracy of anymeasurement of pigment by comparing grayscale images obtained from priorart instrumentation.

The present invention, in one embodiment, uses a multiband filter tosharpen the color resolution of a digital color fundus camera. Usingsuch a filter, it is possible to select red, green, or blue wavelengthsand have the corresponding wavelengths imaged in the corresponding red,green, and blue channels of the color CCD. Such a method is particularlyeffective if the bandpass regions are selected so as to lie between andthereby avoid all or a substantial portion of the above-describedspectral overlap areas between the filter responses of the Bayer filterwithin the camera's CCD. The result in such a case is to effectivelysharpen the spectral response of the apparatus so that accuratemeasurements can be made.

One embodiment of a fundus camera-based system for practicing themethods of the present invention is shown in FIG. 3. FIG. 3 is asimplified schematic diagram showing a fundus camera-based system forpracticing the present invention, and an exemplary placement of thefilters used for macular pigment imaging. In FIG. 3, 301 is the lightsource, which is generally a halogen light for viewing and a xenon flashlamp for photography. Light produced travels through condenser 302 andrelay 303 lenses to reach the multiband passband filter at 304. Thisfilter can be shifted into the optical path as need for macular pigmentphotography. The light is reflected by mirror 305 through relay lens 306to the holed or fenestrated mirror 307. This directs light toward theeye through objective lens 308 into the eye 309. Light returning fromthe eye is focused by the objective lens through the hole in mirror 307.The barrier filter 310 is placed such that it can be brought intoposition to block unwanted wavelengths of returning light if needed. Thelight passes through focusing 311 and imaging lenses 312 and passesthrough the area occupied by a switching mirror 313. When the operatoris viewing the fundus, light is directed to mirrors 314 and 315 throughthe eyepiece 16 to the operator's eye 317. When a photograph is takenthe switching mirror position drops down and light is reflected tomirror 318 through relay lens 319 to the image-recording device 320.While this conceivably could be a film camera, in practical use it is aCCD camera connected electronically 321 to a computer and display 322.Images can be recorded digitally by frame capture.

Preferably, multiband filter 304 is a single filter having multipleoptical bandpass regious. Such multiband bandpass filters aremanufactured using ion beam sputtering and can obtain increasedbrightness and very accurate selection of bandpass wavelengths. Thesefilters are commonly used for fluorescence microscopy. They areoccasionally used for color photography. These filters can be designedto allow multiple bands of selected wavelengths to pass with blocking ofunwanted wavelengths to equal or exceed 5 OD.

In the reflectance method used in one embodiment of the invention,multiband filters are used to record specific wavelengthssimultaneously. The filter used for macular pigment imaging preferablyhas narrow bandpass regions near 465 nm, 535 nm, and 630 nm. Thesecorrespond to blue, green, and red visible wavelengths, and arerepresented in the blue, green, and red channels of the fundus camera,respectively. This allows simultaneous recording of three bands ofinformation at no increase in cost for detectors. When combined, theresultant image is a full color image that looks remarkably like aregular color picture. The color information can be easily deconstructedinto component channels. Since each of the separate channels was takenat the same time with the same camera and detector they have the samefield of view, magnification, and lighting. This permits ratiometricevaluation without the need for any preliminary image registrationsteps.

FIG. 4 shows the idealized wavelength selection of the preferredembodiment multiband filter described above, with red green and bluebandpass regions 402, 404 and 406 respectively. FIG. 5 shows the filterbandpass regions of FIG. 4 overlayed on the spectral diagram of FIG. 2.It can be seen that in this representation, bandpass region 406 capturesthe (blue) peak of the macular pigment absorption curve 214, whereasbandpass region 404 captures a green signal including very little ornone of the spectral region of macular pigment absorption.

To calculate the amount of macular pigment it is customary to calculatea ratio of the reflectance of the two wavelengths at two locations inthe fundus, one within the region of the macula and the other in theperiphery area outside of the region where there is deposition of themacular pigment. The optical density of the macular pigment iscalculated as the log(macula reflλ₁/macula reflλ₂)−log(peripheryreflλ₁/periphery reflλ₂), where refl is the reflectance at either thelong wavelength (λ₁) or the short wavelength (λ₂).

The invention also concerns methods to display the spectral information.The typical fundus camera acquires a high resolution photograph of theeye that encompasses an angular measurement of approximately 50 degrees.The image may be acquired by a computer and processed digitally. Onlythe central portion of this is needed, and a rectangular area isselected. In one embodiment of the present system, as shown in FIG. 6, arectangle 601 measuring 1640×1435 pixels per side is used whichrepresents an area of approximately 8×7 mm. A circular region 602 in thecentral 1230 pixels is used to calculate the ratio macula reflλ₁/maculareflλ₂ and the remaining portion 603 of the rectangle is used tocalculate periphery reflλ₁/periphery reflλ₂. The green channelinformation is divided by the blue channel. This produces a grayscaleimage where the macular pigment appears bright. However there may be ageneral level of gray visible outside of the region of interest causedby scatter, etc. The circular area 602, which corresponds to a 6 mmdiameter circle centered on the posterior pole of the eye is measuredwithin this square. The portion of the rectangle 603 not included in thecentral 6 mm circle is assumed to have negligible amounts of macularpigment. The mean grayscale level of this region is measured. Thisgrayscale level is subtracted from the individual values of the pixelsin the entire square. The resultant image has black regions where thereis no macular pigment and varying levels of gray where there is pigment.

The human eye is not good at differentiating small changes in grayscaleso, preferably, the images are given a pseudocolor. This helpsdifferentiate small differences in the grayscale upon visual inspection.In one embodiment, as illustrated in FIG. 7, the pseudocolor is mappedto the grayscale by a method wherein the color assigned varies accordingto the underlying grayscale value, which in turn varies with the amountof macular pigment. The colors may be arranged in a gradient. In oneembodiment, this gradient generally follows the order of the spectrumfrom red to violet, except that, because it is difficult to see thedifference between blue, indigo and violet, the gradient was createdsuch that the colors vary from black to blue for absent or nearly absentlevels of pigment. Thus, the gradient map colors in that embodimentstart at black and then range the colors of the rainbow from blue togreen, yellow, orange and red, with white added to represent thegreatest value, for increasing amounts of the underlying grayscalevalue. The mean value of the central 500 microns of the macula (604 inFIG. 6) is measured. This grayscale value is compared with a nomogram ofvalues. The grayscale value of the person being tested is put into oneof five groups based on the quintiles of normal distribution of theamounts of macular pigment in the general population. White is assignedto those in the highest quintile, red the second quintile, orange themiddle quintile, yellow next, and green the lowest quintile. The rest ofthe pseudocolor map follows from the highest color down. This allows theexaminer to rapidly estimate the amount of macular pigment present ascompared with normative data.

In addition, each channel of output from the camera can be viewed inisolation to gain additional information about the ocular fundus.Alterations in the nerve fiber layer of the eye can often be bestvisualized by using short wavelengths of light as obtained with the blueportion of the filter. The green channel generally supplies the highestspatial resolution because there are more green sensitive elements inthe color CCD. The red channel affords better visualization of deeperstructures within the retina. Ratiometric comparisons between variouschannels, or composites of more than one channel can improvevisualization as required. For example, an average of the red and greenchannels may be performed, and the average used to normalize the bluechannel by dividing the blue channel with the red/green information,thereby improving contrast.

Accordingly, the present invention provides a multiwavelength,topographic analysis of macular pigment, acquired with a singlephotograph, using commonly available equipment and commerciallyobtainable filters, produces highly useful and accurate data regardingmacular pigment, and provides representations of the data in forms mostadvantageous for visual inspection by the opthalmologic practitioner.

It should be apparent from the foregoing, therefore, that the presentinvention achieves its objects and overcomes many of the shortcomings ofthe prior art.

Although the present invention has been described with specificembodiments, a variety of changes, substitutions, variations,alterations, and modifications in accordance with the principles andapparatus described herein may be readily suggested by this disclosureto one skilled in the art. It is intended that the invention encompassall such changes, substitutions, variations, alterations, andmodifications as fall within the scope and spirit of the followingclaims.

1. A method for imaging macular pigment in a patient comprising a)taking a digital color image of the macula using a digital color cameraproviding a plurality of discrete, color-associated grayscale signalswithin a first set of color pass bands; and b) interposing in the lightpath of said camera at least one filter providing a second set of colorpass band regions, wherein at least two of said second set of pass bandregions are chosen to be within and narrower than two of the pass bandregions of said first set of pass band regions, and one of said at leasttwo of said second set of pass band regions is chosen to beapproximately centered in the absorption band of macular pigment.
 2. Themethod of claim 1 wherein said at least two of said second set of passband regions each lies substantially outside of any area of spectraloverlap between the respective pass band regions of said first set ofpass bands.
 3. The method of claim 1, wherein said digital color cameracomprises an array of monochromatic light detectors and a filter arrayhaving individual filter elements corresponding to individual detectorelements.
 4. The method of claim 2 wherein said filter array is a Bayerfilter and said at least one interposed filter is a multiband filterhaving bandpass regions centered near 465 nm, 535 nm, and 630 nm, andwherein each of said bandpass regions lies substantially outside of anyarea of spectral overlap between the respective pass band regions ofsaid first set of pass bands of said filter array.
 5. The method ofclaim 4 wherein said multiband filter comprises a single filter.
 6. Themethod of claim 5 wherein said multiband filter is manufactured by ionsputtering.
 7. A method for measuring macular pigment comprising a)performing the method of claim 1 and obtaining in connection therewith,grayscale data from said grayscale signals, at least two wavelengths λ₁and λ₂, the second of which wavelengths being approximately centered inthe absorption band of macular pigment and the first of said at leasttwo wavelengths being a longer wavelength in the green portion of thespectrum; calculating the ratio of the reflectance of said twowavelengths at first and second locations in the fundus, the firstlocation (macula) being within the region of the macula and the secondlocation (periphery) being outside of the region where there isdeposition of the macular pigment; and b) calculating the opticaldensity of the macular pigment as the log(macula reflλ₁/maculareflλ₂)−log(periphery reflλ₁/periphery reflλ₂), where refl λ₁ is thereflectance at the longer wavelength (λ₁) and reflλ₂ is reflectance atthe shorter wavelength (λ₂), at the macula and periphery locations,respectively.
 8. The method of claim 7 used to create a grayscale imageby a series of steps further comprising a) selecting a generallyrectangular area of said image, corresponding to an area ofapproximately 8×7 mm of the ocular fundus enclosing the macula; b)selecting a generally circular region of approximately 6 mm diameterapproximately centered in said generally rectangular area; c)designating said generally circular area as the first (macula) locationin the calculation of claim 7; and d) designating the portion of saidgenerally rectangular area outside of said generally circular area asthe second (periphery) location in the calculation of claim
 7. 9. Themethod of claim 8 further comprising subtracting the mean grayscalevalue measured in said generally rectangular area outside of saidgenerally circular area from the grayscale values measured at each pointthroughout said generally rectangular area.
 10. The method of claim 9adapted to create a pseudocolor image by a series of steps furthercomprising assigning visual indicia selected from a set of visualindicia to a set of grayscale values in an image and displaying saidvisual indicia at the locations of the corresponding grayscale values inaccordance with said selection of visual indicia.
 11. The method ofclaim 10 wherein said visual indicia are colors.
 12. The method of claim11 wherein the color selections in accordance with claim 10 are from aset of colors regarded as ordered in a gradient and wherein each colorsare assigned to grayscale values in accordance with the position of therespective colors in said gradient.
 13. The method of claim 12 whereinsaid gradient ranges from black to blue for grayscale valuescorresponding to absent or nearly absent levels of macular pigment, andranges over other colors for increasing amounts of the underlyinggrayscale value.
 14. The method of claim 13 wherein said other colorsrange from green, yellow, orange, red, and then white for increasingamounts of the underlying grayscale value.
 15. The method of claim 12further comprising normalizing said image with respect to a generallyrepresentative set of subjects by a series of steps further comprisinga) measuring the mean grayscale value of approximately the central 500microns of the macula; b) comparing said grayscale value with a nomogramof corresponding values measured from a generally representative set ofsubjects and assigning said grayscale value to a group based on itsposition with respect to said nomogram; c) selecting the color torepresent the highest grayscale value in said image in accordance withsaid assigned group; and d) scaling the selection of said other colorsin said color gradient according to said highest color value.
 16. Themethod of claim 15 wherein the grayscale value of the person beingtested is put into one of five groups based on the quintiles of normaldistribution of the amounts of macular pigment in the generalpopulation, and wherein white is assigned to represent the highestgrayscale value for those in the highest quintile, red for the secondquintile, orange for the middle quintile, yellow next, and green for thelowest quintile, and the remainder of the pseudocolor map follows fromsuch color in accordance with the color gradient otherwise selected inaccordance with claim
 12. 17. The method of claim 1 further comprisingperforming an opthalmologic analysis based upon said a visualrepresentation of at least one of said grayscale signals.
 18. The methodof claim 17 wherein said analysis is based on a visual representation ofa single one of said grayscale signals.
 19. The method of claim 18wherein said single grayscale signal corresponds to a blue pass band andsaid analysis comprises an analysis of nerve fiber.
 20. The method ofclaim 18 wherein said single grayscale signal corresponds to a greenpass band and said analysis comprises a spatial analysis.
 21. The methodof claim 18 wherein said single grayscale signal corresponds to a redpass band and said analysis comprises analysis of structures within theretina.
 22. The method of claim 17 wherein said analysis is based on avisual representation derived from a calculation based on at least twoof said grayscale signals.
 23. The method of claim 22 further comprisingaveraging grayscale signals corresponding to red and green pass bandsand using said average to normalize a grayscale signal corresponding toa blue pass band by dividing the data from said blue pass band by saidred/green average data thereby improving the contrast provided by saidblue pass band data.
 24. Apparatus for imaging macular pigment in apatient comprising a) a digital color fundus camera providing discrete,color-associated grayscale signals within a first set of color passbands and being adapted to hold a user-selected filter; and b) at leastone filter interposed in the light path of said camera providing asecond set of color pass band regions, wherein at least two of saidsecond set of pass band regions are chosen to be within and narrowerthan two of the pass band regions of said first set of pass bandregions, and one of said at least two of said second set of pass bandregions is chosen to be approximately centered in the absorption band ofmacular pigment.
 25. The apparatus of claim 24 wherein said at least oneof said second set of pass band regions also lies substantially outsideof any area of spectral overlap between the respective pass band regionsof said first set of pass bands.
 26. The apparatus of claim 24, whereinsaid digital color fundus camera comprises an array of monochromaticlight detectors and a filter array having individual filter elementscorresponding to individual detector elements.
 27. The apparatus ofclaim 25 wherein said filter array is a Bayer filter and said at leastone filter is a multiband filter having bandpass regions centered near465 nm, 535 nm, and 630 nm, and wherein each of said bandpass regionslies substantially outside of any area of spectral overlap between therespective pass band regions of said first set of pass bands.
 28. Theapparatus of claim 27 wherein said multiband filter comprises a singlefilter.
 29. The apparatus of claim 28 wherein said multiband filter ismanufactured by ion sputtering.
 30. The apparatus of claim 24 furthercomprising a processor having an interface to receive and process outputfrom said fundus camera.